1. Field of the Invention
The present invention relates to a zoom lens system, particularly preferable for use as an image-taking optical system of a video camera, a digital still camera, and the like.
2. Description of the Related Art
In recent years, as enhanced functionality and a smaller size are achieved in an image-taking apparatus such as a video camera and a digital still camera which uses a solid-state image-pickup device, both higher optical performance and a smaller size are needed in an image-taking optical system used in such an image-taking apparatus.
A known zoom lens (a variable magnification optical system) used in such an image-taking apparatus is of a four-unit structure with a positive, a negative, a positive, and a positive refractive powers. The zoom lens has, in order from an object side to an image side, a first lens unit with a positive refractive power stationary during zooming and focusing, a second lens unit with a negative refractive power which moves on an optical axis to provide variable magnification, a third lens unit with a positive refractive power stationary during zooming and focusing, and a fourth lens unit with a positive refractive power which moves during zooming and focusing. For example, each of Patent Document 1, Patent Document 2, and Patent Document 3 discloses a zoom lens in which a second lens unit provides variable magnification and a fourth lens unit corrects variations in image surface position associated with varied magnification. The zoom lens disclosed in each of Patent Documents 1 to 3 has an aperture stop disposed immediately before a third lens unit.
(Patent Document 1)
Japanese Patent Application Laid-Open No. 7-270684 (No. 1995-270684) (corresponding to U.S. Pat. No. 5,963,378)
(Patent Document 2)
Japanese Patent Application Laid-Open No. 7-318804 (No. 1995-318804) (corresponding to U.S. Pat. No. 5,963,378)
(Patent Document 3)
Japanese Patent Application Laid-Open No. 11-305124 (No. 1999-305124) (corresponding to U.S. Pat. No. 6,166,864)
In recent years, with a smaller size of an image-taking apparatus such as a digital still camera and a video camera and a higher number of pixels of an image-pickup device, both higher optical performance and a smaller size are needed in an image-taking optical system used in such an image-taking apparatus. In addition, in a video camera, recording of still images of high quality is desired, and higher optical performance is required than in an image-taking optical system intended only for taking moving images. At the same time, a lens system with the size maintained at the current level or a smaller size is needed.
Generally, in a zoom lens, increasing a refractive power of each lens unit can reduce a movement amount of each lens unit for providing a predetermined zoom ratio to achieve a higher zoom ratio and a smaller overall length of the lenses.
Simply increasing a refractive power of each lens unit, however, presents a problem that exacting tolerances are needed in manufacture. For example, if relative axial displacement occurs in lenses within the third lens unit in the four unit zoom lens with the positive, the negative, the positive, and the positive refractive powers as described above, image quality is significantly degraded.